Assembly line balancing problem (ALBP) is defined as assigning work to workstations in order to perform work in a minimum cycle time and to distribute work so as to keep nearly equal utilization of each resource. ALBP is important problems in apparel manufacturing. Apparel manufacturing is man intensive manufacturing environment. In a sewing line, each task should be performed on a standard time. The standard time is the time required to perform a specified task using a prescribed method. Operator Skill level (OSL) has a great effect on the ability of the sewing line to complete jobs on the committed time. A skill operator is this who is able to perform repetitive tasks on the standard time. Due to different OSL, the total completion time of an order quantity is variable. In this paper a shirt sewing line is studied to notice the effect of OSL on jobs distribution among workstations. A sewing line is simulated and a multi-objective simulation optimization approach is used to determine the best number of workstations in each group of tasks to minimize work cycle time and to keep nearly the same level of operators utilization. The results show how OSL affects solution of ALBP.